Relay Coil Free-Wheeling Diodes

Maybe this can be moved to the technical library if it's deemed worthy...

I was doing some self-imposed day job work today verifying some new circuit boards I just got in. These were relay boards, where relay as in something with a coil and contacts. Tractors have these all over the place. My 4520 has a glow plug relay. Some people with electrically actuated hydraulics for grapples have switches that run relays that run the grapple.

The win of a relay is that you can take a small switch and power the coil with a small current. The contacts on the relay can then power something that needs a big current. For example, you can put .08 amps through a coil and the contacts can then pass 20 amps through the coil that turns on the hydraulics. Note the .08 amps is also written as 80 mA where the "m" stands for milli or 1/1000. You would say the coil has "80 milli-amps".

So the win here is that the switch on your joystick can be rated for a much lower current (say 1/2 amp) even though you're working something that takes 20 amps. This also means the wiring up into the joystick can be a small stranded wire because it does not have to pass 20 amps.

There's some funny business that goes on with the relay coil. You see, when you energize a coil and it builds up a magnetic field, you are storing energy in the magnetic field. Yes, the magnetic field is pulling in the contacts to make the relay work but there is energy stored in the field. When you let go of the switch that powers the coil, that energy has to go somewhere.

The universe gets really upset if you try to loose energy. If you've got some (like in the coil) it has to go somewhere. Eventually, that energy is turned into heat. In fact, the coil has resistance and that resistance turns some of the energy into heat. Nothing you could feel, since the power driving the coil is a bit less than a watt (.08 Amps x 12 Volts = .96 Watts). So how does the coil get rid of the energy? Well, the next thing about coils and the universe is that the current that flows in the coil likes to keep flowing now mater what.

So the coil will very quickly change the voltage across the terminals in a desperate attempt to keep the current flowing though the coil. Eventually, the voltage gets so large that something arcs (usually the contacts on the switch) and the energy is dissipated in the arc. Of course this is a bit hard on the switch contacts, but they are sort of designed to take this.

So when you have a coil energized and then turn off the switch, the voltage quickly takes off in the opposite direction of what voltage was applied in a frantic effort to keep the current flowing. If you had +12 on the coil (with the other lead at ground or battery minus), that terminal will quickly head off down through ground and want to keep going. You can get -100 or more volts in a few millionths of a second.

So like I was saying, a lot of the time the negative-moving voltage on switch release just goes to a very high voltage and you get a small arc on the switch and life moves on.

But sometimes you might want to protect the switch, or use a really small switch that would not last long with all that mico-arcing, or you might be driving the relay with some piece of electronics that can't take a -200 volt joke. So a common practice is to put a free-wheeling diode across the coil. You can see this in the schematic. Not that the cathode (bar end) goes to plus, and the anode (the arrow end) goes to ground. So the diode is reverse biased when you apply voltage and no current flow. From a schematic point of view, a diode only lets current flow in one direction, and that's the direction the arrow points.

When you release the switch and the voltage on the coil screams negative, just as it gets a bit below ground (about .75 volts below ground), the diode starts to conduct the current in the coil. Now remember that the coil gets all upset if the current in it takes an abrupt change. Well, now that the coil is at negative .75 volts and the diode conducts, the current is flowing. The coil is happy . So it stops the screaming run of voltage to some unknown negative value.

Now we still have to get rid of the energy, but the coil has resistance and with the current flowing through the resistance of the coil, the energy slowly dissipates and turns into heat in the coil. Again, it's not much heat- you couldn't feel it if you tried.

Now lets look at the picture of the voltage on the coil (the 2nd picture). You can see where ground its on the left by the little '1' and ground symbol. You can see the coil going up to about +12 volts. The little "2.00V/" by the yellow '1' in the upper left means the scale is 2 volts per division vertically. You can also see about 2/3 of the way over on the top the characters "5.00mS/" which means the the horizontal time scale is 5 milliseconds (or .005 seconds) per division. So we see the coil energized for about 10 mS, then the power is shut off. You can see a little contact noise during the turn off period. Once the coil switch is really, really, off, the voltage dives negative. It goes about 1V below ground. Then, you can slowly see it approach ground, and then kick up to ground. This process takes about 22 mS or .022 seconds. During this time, the energy in the coil is going away. I wanted to write disapating. but I'm so far off on the spelling that the spell checker is laughin' at me .

So the simple diode across the coil has kept the voltage from screaming to a negative value and our contacts are saved!

Next, lets look at how fast the voltage moves and talk some "Diode Details" .

Wake up! This next picture show just how fast "screamin' fast" is when that coil decided to go negative in it's quest to keep the current flowing. The vertical scale is still 2V per division. This is a close up of when the switch opens and the voltage takes off.

What's different here is the horizontal time scale. It is 100 nS per divisions. That would be one ten millionth of a second per division. This is about the same amount of time it takes for light to travel 100 feet.

So we see that in about 300 nS (that's nanoseconds or billionths of a second) the coil voltage goes from +12V to -1 volt. It stops at -1 volts because the diode kicked in and started to conduct the current.

This time frame is also why sometimes when a motor kicks out you can hear a click on the radio or see a burst of noise on the old analog TV. The 300 nS corresponds to a frequency of about 3 MHz, so on the AM radio band you'd hear a click from the relay coil kicking out.

But none of this is the point of the story...

It turns out that the name "diode" is a very general term, kind of like "hydraulic valve". There are all kinds of different diodes, and one of the specifications for a diode is it's speed. You want a high speed diode that can start conducting current in a few 10ths of a billionth of a second. A common mistake that people make is to take a diode designed to rectify AC (as in 60 Hz AC from a transformer) and make DC. These are not high speed diodes. A common part number here is a 1N4004. Don't use the 1N4004 as a free-wheeling diode.

I like a diode called a TVS or Transient Voltage Suppressor diode. The symbol for it is drawn like a simple diode, but with little wings on the cathode bar. They have voltage ratings, and something like a 15 or 18 volt rating works fine.

Now lets look at the next schematic picture. What you see here is called back to back zener diodes. They will conduct if the voltage across them exceeds 16 volts no matter what the polarity. Well, ok, a part rated at 15 volts will start to conduct at 16. Zeners are rated at the maximum safe working voltage. So if the +13 going into our relay spikes up above 16 volts the diode will start to conduct current, and if the coil starts the process of screaming to a negative voltage they will conduct when there is -16 volts or so.

The nice thing about back to back zeners is that there is no polarity. You can buy back to back zeners in a single package. No way to install it wrong, there's no polarity . You just put it across the coil.

Now lets look at the time waveforms for this. You can see that we are at a 5V per division vertically, and at 1 millisecond (or 1/1000 of a second) horizontally. You can see the +13 applied to the coil. About 4 mS (or .004 seconds) later, the switch is turned off and the coil voltage starts to scream to some negative value in that desperate effort to keep the current flowing and not upset the universe.

Then, you see the back to back zener kicking in at about -16 volts. In less than a millisecond, the energy is about gone and in two milliseconds the coil voltage goes to zero as all the energy is dissipated. You may recall that with a simple diode across the coil, it took about 22 mS to dissipate the energy. But with the back to back zeners, it takes less than 2 mS.

The reason for this is that with just the simple diode, the majority of the current was being dissipated in the coil. But with the back to back zeners, the energy is being dissipated in the diode. Instead of coil voltage being around 1 volt with the simple diode, it is about -16V with the back to back zeners. So with that bigger voltage the energy in the coil dissipates faster. Our switch contacts are OK, since it takes much more than 16V to arc over.

Now the back to back zeners are a win here because they keep from beating up the contacts on your coil switch. It's an easy install and you can't get it backwards. But there is one more win.

Well, with the simple diode the energy takes about 22 mS to dissipate. The contacts that are being held in by the coil release slowly. If there is any arcing on the contacts, the slow moving contacts have longer to arc. Now remember we're dealing with thousands of a second here, but it makes a difference.

With the back to back zeners the energy dissipates fast and the coil drops the moving armature that has the contacts faster. The spring in the relay snaps back faster, the contacts separate faster and any arcing happens in less time so there is less heat and less damage.

It's like if you pick up something hot, if you quickly open your fingers and throw it away you get less burn than if you slowly open you fingers just until the object drops. So the back to back zeners can increase the life of the contacts of the relay.

There are lots of circuits out there for driving relays. Many of them deal with contact closure and drop times. I've seen 100 volts applied to a 12V relay to pick it up fast with no contact bounce. Then, the voltage is dropped to 10 volts to hold it in. I've also seen circuits designed to really suck the power from the coil for a fast drop.

Now right about now a lot of you are thinking "Boy, that Pete sure likes to worry about nothing, I've had my grapple switch run for years with no problems". And that may well be true. Relay coils and switches are typically rated for 100,000 to 1 million uses. So you might only have 10,000 uses on yours and be OK- only 1/10 of the way through the life of it all.

But if you've every had problems with a switch or contact burn out, this back to back zeners across your relay coil might be something to try. Let me know what you think- have you every had problems, or is the number of cycle times of these switches and relays on a tractor so small that we never get to see them beat up ?

Now I got one more post and then I'm done. Stick with me...:empathy3:...

This last schematic is a simplified version of what I use when I have "big" relays with coil currents over 30 mA. You can see the coil with the back to back zeners on it. There is also a transistor driver that uses the PNP transistor to supply voltage to the coil. When power is dropped, that transistor turns off. The win is that the negative voltage spike we saw with the back to back zeners never gets sent back out on the wire back to the switch. This circuit is what I have on the circuit boards I made that have relays. The win is that whatever is driving the relay coil has no idea it's driving an inductive circuit with that negative kick when you turn it off. For long wires driving relays, like you might have in a house, there is no interference to audio systems or radios since the negative spike is localize to within an inch of the coil.

Now if this doesn't make sense, don't worry, and I don't think this is needed on a tractor but I toss it out there for those who can read a simple schematic just to give you something to think about.

Ok, that's it. If you've got switch contact wear problems here's your answer- the back to back zener. If you've got pre-mature contact wear, here's something to try to get more contact life. If you're worried that contact bounce in your switch might be cramming small -100 volt spikes back into your tractor's +13 volt line, this can keep the spikes at bay.

If there is interest, I'll post about how to get some of these parts. You could also get some other good electrical stuff I've talked about like heat shrink tubing and solder at the same time to build up your electrical arsenal. Let me know if anyone has had problems with this sort of thing, or if I'm just having a little personal obsessive compulsive moment here....